HP-67 low power indicator circuit?

[John Garza (3665)]

Ah... no wonder I could not find the circuit. I was curious how the 67 did it as I'm thinking about building a low power indicator in a TI-58C. To TI's credit, they finally got around to a continuous memory model. However, the only sure sign of low power is a garbled display and by then it's too late. Defeats the whole purpose of continuous memory. I think I'll use a TL431 with a voltage divider to make it sense 3V or so. The 58 seems to work fine all the way down to 2.45V. A small LED in the corner of the display a la 67 will be the indicator.